The known resonant-mode power supplies contain switches, usually in the bridge or half-bridge configuration composed of controllable semiconductor devices, most often transistors, supplied from a voltage-source power supply, in the bridge or half-bridge diagonal whereof is connected a resonant circuit with a load connected by means of an output transformer.
In the Polish patent application P-313150 there is described a resonant-mode power supply which maintains a constant quality factor of the resonant circuit independently from the load. The resonant-mode power supply incorporates a quality-factor limiter composed of a transformer whereof the primary winding is connected in parallel with the resonant circuit capacitor whereas the secondary winding of said transformer is connected to the power supply source to allow feeding the excess energy from said capacitor back to the source. The distinctive feature of this resonant-mode power supply is the capability of correct operation with both shorted and open output circuit.
In the Polish patent application P-339678, a capacitive voltage divider with the equivalent capacity equal to the required resonant circuit capacity is employed instead of a transformer. By means of connecting a diode limiter between the current switches power supply bus and the common node of the capacitive voltage divider capacitors the voltage amplitude at this point was limited, thus energy recirculation and limitation of the series resonant circuit quality factor were achieved.
A technical drawback of the power supplies with energy recirculation according to patent specifications P-313150 and P-339678 is that in both of them where the load decreases, i.e. the load resistance increases, also the series circuit current decreases and its waveform become differ from the desired sinusoidal shape. Another major technical drawback of the solution described in the patent application P-313150 is that the energy recirculation circuit necessitates the use of a transformer of nearly the same power as that of the output transformer. The solutions described in the state-of-the-art literature utilize an additional winding of the output transformer connected through a rectifier to the power supply source in order to stabilize the output voltage or limit said output voltage where the output circuit becomes open.
From the US patent application US 2006/0227577 there is known a resonant converter intended for operation with an inverter. The converter enables transformation of fluctuating and relatively low voltages, obtained from renewable energy sources, to the level required by power grid. The converter comprises a parallel resonant circuit to which direct-current power is input from a low-voltage direct-current power supply by means of switching elements. DC-AC conversion is performed by means of zero-voltage switching. The high-frequency transformer whose primary side is connected to the parallel resonant circuit provides electrical isolation and generation of high voltage. The secondary side of the transformer is connected with a rectifier trough a series resonant circuit. The converter provides output voltage of 450V with output voltage changes of about 25-30%. The described converter structure is sensitive to rapid load changes. If at maximum output power a sudden disconnection of load occurs the energy stored in the resonant circuit, which generally is much larger than energy transmitted to the load during a single commutation cycle, may produce currents in the commutation circuit exceeding permissible values.
The aim of the invention is to develop a resonant-mode power supply for transformation of direct-current voltages, characterized by sinusoidal currents in the resonant circuit independently of the load and by high immunity to rapid changes in the output power.